I. Field of the Invention
This invention relates to unsaturated polyesters, their preparation and the use of the unsaturated polyesters in the manufacture of various articles.
II. Description of the Prior Art
In commonly assigned French Pat. No. 1,336,751 of July 12, 1962 and corresponding U.S. Pat. No. 3,303,240 issued Feb. 7, 1967, colleagues of the inventors herein have disclosed self-extinguishing polyesters obtained from the esterification of polyols derived from NN' substituted polyhalogenated diphenyls and the copolymers prepared from these polyesters. The primary interest of these products now commercially supplied under the trade names ERVAPON and ERVADIOL by Societe Plastimer lies in their excellent self-extinguishing characteristics which are due to the presence of halogen atoms substituted upon the phenyl radicals of the diol. These products are also of interest for their good resistance to corrosion from such reactant chemicals as acids and bases. On the contrary, their resistance to solvents, and in particular chlorinated solvents, is not always sufficient even though they are clearly better in this respect than known and conventional polyesters obtained from diols having relatively short chains.
Subsequent studies carried out by the inventors with polyesters derived from maleic acid according to this patent have shown that in addition to maleic groups, there is also a small amount of fumaric groups present, on the order of from 20 to 30% according to measurements carried out by means of nuclear magnetic resonance techniques. These fumaric groups result from the partial isomerization of the unsaturated acid in the course of the esterification reaction.
It has already been pointed out in the literature, in particular, in French Pat. No. 1,392,411, that the polyesters derived from the reaction of aliphatic polyols with maleic anhydride, an anhydride employed alone or in mixture with other anhydrides or acids is presently the most utilized diacid for the preparation of polyesters, can be converted into polyesters of improved properties by isomerization in situ of the maleic into the fumarate. This was not surprising since it was also known that the esters of fumaric acid provide superior properties to the maleic homologues. However, the commercial use of fumaric acid for the preparation of polyesters is limited due to the price of this material (fumaric acid is generally prepared from maleic acid), the yield of esterification is relatively low and above all, due to the operational difficulties resulting from the higher melting point of fumaric acid.
In the case of known polyesters, the isomerization of the maleate into the fumarate can be accomplished by means of various known and conventional processes, for example, by the action of bromine under the influence of light or by the use of iodine or sulfur as a catalyst. However, according to French patent 1,392,411, the process which provides the best results is that which comprises utilizing secondary cyclic amines as catalysts. These catalysts simultaneously accelerate esterification and isomerization. Piperidine and its derivatives have been found to be particularly advantageous for this purpose.